Many of the malarial asexual blood-stage antigens thus far identified are associated with cytoplasmic inclusions and/or accumulate within the parasitophorous vacuole of the infected erythrocyte. Such antigens may be shed or secreted into the plasma of infected individuals or into the supernatant medium of in vitro cultures, especially upon schizont rupture. These soluble antigens have been historically referred to as exoantigens (Wilson et el. 1969). Since exoantigens are known to occur in the circulation of patients with malaria, they may be responsible for excess production of tumor necrosis factor (TNF), a mediator that is thought to play a central role in the pathogenesis of the disease (Scuderi et el. 1986; Kwiatkowski et al. 1989, 1990; Taverne et al. 1990a,b; Playfair et al. 1991; Mendis 1992). TNF may promote cytoadherence and sequestration of Plasmodium falciparum-infected erythrocytes (Grau et al. 1989; Molyneux 1990) and when produced in excessive levels may predispose to cerebral malaria and a fatal outcome (Kwiatkowski et el. 1990). In addition, malarial fevers as a result of P. falciparum and P. vivax infections may be mediated, at least in part, through paroxysmal TNF release associated with schizont rupture (Kwiatkowski et el. 1989; Karunaweera et al. 1992; Mendis 1992). It has been suggested that immunization with an exoantigen(s) might provide a means of protection against the clinical effects of malaria and of generating anti-disease immunity by reducing cytokine production (Bate et el. 1990; Playfair et el. 1990). Such an exoantigen-based, anti-disease vaccine would minimize the clinical manifestations of P. falciparum malaria, thereby extending life until the development of solid naturally-acquired immunity.
The present invention relates to the immunogenicity and antigenicity of novel peptides and a unique synthetic peptide hybrid (SPf70) derived from internal chymotryptic fragments of a major 70 kDa P. falciparum (Indochina I/CDC) schizont protein (Shamansky et al. 1985; Braun-Breton et al. 1986). It has been suggested that the 70 kDa polypeptide is a degradation product of a 120 kDa schizont membrane protein, with the 70 kDa protein increasing in amount at the time of merozoite release/reinvasion (Braun-Breton et al. 1986). This antigen was selected for study because of its immunogenicity and efficacy in inducing partial protective immunity in susceptible Bolivian Saimiri monkeys (James et al. 1985). Partially purified (enriched for the 70 kDa antigen) supernatant fluids of P. falciparum Indochina I and Geneve/SGE-1 strains conferred significant clinical protection of squirrel monkeys against challenge with the homologous Indochina I strain and a moderate degree of heterologous strain immunity. Subsequently, monospecific rabbit antibodies to the 70 kDa polypeptide were shown to have schizont specificity by immunofluorescence, and approximately 50% inhibition of P. falciparum growth after 72 h of in vitro culture.
Synthetic peptides have been shown to be powerful tools for the seroepidemiology and diagnosis of malaria (WHO Scientific Group, 1989). From the standpoint of immunodiagnosis, it is desirable to utilize peptides that are both specific and conserved between different parasite strains (Chizzoline et al., 1989; Peterson et al., 1989). Using defined antigens various seroepidemipologic surveys have shown that in malaria-endemic areas, antibodies to selected parasite proteins, such as the circumsporozoite (CS) protein, Pf155/RESA and MSA-1, increase with age and exposure to malaria parasites (Campbell et al., 1987; Chizzolini et al., 1988; Petersen et al., 1989). However, reports have indicated that individuals in endemic regions, at equal risk for malaria, show intrinsic differences in their ability to generate antibodies against specific parasite proteins (Chizzolini et al., 1988; Rosenberg and Wirtz, 1990). Synthetic peptides have been of value in monitoring antibody levels in areas where malaria transmission is seasonal and/or unstable (Esposito et al., 1988; Deloron et al., 1989; Deloron and Cot, 1990). In general, such studies have shown that antibodies to blood-stage parasites do not persist in the absence of reexposure. (Tapchaisri et al., 1985; Wijesundera et al., 1990.)